Preheat oven to 350 degrees F. Score zucchini (like you’re dicing an avocado) and scoop out insides into a large bowl.
In a large skillet over medium heat, heat oil. Add chicken sausage and cook until seared, 6 minutes.
Transfer to a large bowl and add breadcrumbs, 1/2 cup mozzarella, fontina, garlic, and basil.
Spoon mixture into zucchini and top with remaining 1/4 cup mozzarella.
Bake until zucchini is tender and cheese golden, 15 minutes.